Problem: The arithmetic sequence $(a_i)$ is defined by the formula: $a_1 = 1$ $a_i = a_{i-1} - 4$ What is $a_{3}$, the third term in the sequence?
From the given formula, we can see that the first term of the sequence is $1$ and the common difference is $-4$ To find the third term, we can rewrite the given recurrence as an explicit formula. The general form for an arithmetic sequence is $a_i = a_1 + d(i - 1)$ . In this case, we have $a_i = 1 - 4(i - 1)$ To find $a_{3}$ , we can simply substitute $i = 3$ into the our formula. Therefore, the third term is equal to $a_{3} = 1 - 4 (3 - 1) = -7$.